1. Field of the Invention
The present invention relates to a non-aqueous secondary electrochemical battery comprising a complex oxide containing lithium for a cathode and a carbon material for an anode, and more particularly, to a non-aqueous secondary electrochemical battery having improved cycle life capabilities, discharge performance, and low temperature performance.
2. Description of the Prior Art
Recently, various kinds of portable or cordless electronic equipment have been developed one after another, and as a power source for driving these equipment, the demand for small-sized and lightweight secondary batteries which have high energy density has increased. In this respect, because of their high voltage and high energy density, non-aqueous secondary lithium batteries have been desired.
As for secondary batteries, nickel-cadmium batteries and lead acid batteries having excellent performance capabilities are commercially available. Therefore, when non-aqueous electrochemical batteries are used as secondary batteries, it is desired that cathode active materials for these batteries have high energy density, that is, high capacity and high potential.
As a cathode active material, a complex oxide containing lithium is well known. For example, U.S. Pat. No. 4,357,215 discloses a battery comprising LiCoO.sub.2 as an active material for a cathode.
On the other hand, U.S. Pat. No. 4,423,125 discloses a non-aqueous electrochemical battery which comprises a carbon material for an anode instead of lithium metals or lithium alloys. Since this battery uses carbon material capable of occluding and releasing lithium ions, it exhibits safety and good cycle life capability.
Moreover, Japanese Laid-Open Patent Publication No. 63-121260 discloses a combination of these U.S. Patents, in which LiCoO.sub.2 and a carbon material are used for a cathode and an anode, respectively.
Generally, when a lithium metal is used for an anode, active dendritic products (dendrites) produced on a surface of the anode are reacted with a non-aqueous solvent to partially decompose the solvent during charging. As a result, charge efficiency is lowered. In this system, the maximum charge efficiency is approximately in the range of 98 to 99%. The same results are also obtained when a lithium alloy is used for the anode.
When a carbon material is used for an anode, it is required that a complex oxide containing a lithium, e.g., LiCoO.sub.2, be used as a cathode. Since lithium metal is not used for the anode, dendrites are not produced on the surface of the anode during charging. As a result, the cathode and anode are kept free from the passage of the dendrites through a separator, which would otherwise cause a short circuit therebetween. The battery can be prevented from igniting or exploding. In this way, the secondary battery which is safe and excellent in cycle life capabilities can be obtained. However, discharge-charge cycles involve a decomposition of a solvent for a non-aqueous electrolyte as a side reaction, which gradually deteriorates the characteristics of the battery. As a result, the charge efficiency can not become 100%.
It is assumed by the inventors that the reason for the above-mentioned side reaction is as follows:
When the carbon material is used for the anode, it is desired that lithium ion alone be intercalated between layers of the carbon material. However, the solvent which is coordinated to the lithium ion is also intercalated between the layers, and then, the solvent is partially decomposed. That is, the solvent whose molecular diameter is large is not intercalated between the layers, so that the solvent is partially decomposed at the entrance thereof.
Examples of a solvent for an electrolyte of the above-mentioned lithium battery preferably include esters such as propylene carbonate and ethylene carbonate. U.S. Pat. No. 4,805,596 also discloses that an ester-based electrolyte is preferably used when LiCoO.sub.2 is used for a cathode.
One of the requirements for a solvent suitable for a lithium battery is a high dielectric constant, that is, capability of dissolving a large amount of inorganic salt which is a solute. The above-mentioned propylene carbonate and ethylene carbonate satisfy this requirement, while these esters have cyclic structures and molecular diameters that are larger compared with the width of the layers of the carbon material. Therefore, when a lithium ion is intercalated between the layers, this type of solvent is partially decomposed, resulting in the partial destruction of the carbon structure.
On the contrary, chain esters are readily intercalated between the layers because of their structure. Examples of the chain esters include dimethylformamide, acetonitrile, diethyl carbonate, and ethyl acetate. However, they also have problems. That is, dimethylformamide and acetonitrile are reactive to lithium. Although diethyl carbonate and ethyl acetate are not reactive to lithium, they have a low dielectric constant, so that they are unable to dissolve a large amount of inorganic salt.
To solve the above-mentioned problems, according to the present invention, a mixed solvent containing a cyclic ester and a chain ester is used as a solvent for an electrolyte, whereby a large amount of inorganic salt is dissolved and a lithium ion with a chain ester is readily intercalated and deintercalated between the layers of the carbon material without the decomposition of the solvent and the destruction of the carbon structure.